PROJECT SUMMARY This K08 Career Development Award details a four-year training program to advance Dr. Samelson-Jones? career goal of becoming an independent physician-scientist focused on leveraging molecular insights of biological processes into new therapeutics for children with congenital blood disorders. During the award period, Dr. Samelson-Jones will continue developing his expertise in coagulation biochemistry, acquire new scientific skills, deepen his knowledge of molecular and cellular immunology, and advance his translational capabilities. Under the guidance of his mentor, Dr. Mortimer Ponz, and co-mentor, Dr. Valder Arruda, these training objectives will be met by a combination of didactic course work, participation in seminar series, research experience, and mentoring by his advisory committee. His advisory committee is composed of world- renowned scientists with extensive mentoring experience and diverse and complementary scientific expertise including Drs. Sriram Krishnaswamy, Rodney Camire, and Michael Milone. The scientific proposal is aimed at addressing the current limitations of therapies for hemophilia B (HB). HB is due to an inheritable deficiency in coagulation Factor IX (FIX) activity. The current standard-of-care in the developed world for HB is to replace the missing FIX with intravenous infusions, but there are also several ongoing early phase gene therapy trials. Recently, it was demonstrated that the incorporation of the hyperactive FIX variant, R338L into a gene therapeutic partially mitigates previously identified efficacy limitations without additional safety concerns. This result emphasizes the potential of hyperactive FIX variants as gene, protein, or cell therapeutics for HB. Dr. Samelson-Jones? proposal focuses on identifying and carefully characterizing new FIX variants with increased activity. He has developed a rational strategy to identify new hyperactive FIX variants by specifically testing substitutions at positions that are 1) evolutionarily conserved; 2) absent from the HB database; and 3) structurally important for protease function. This approach has already identified several new hyperactive FIX variants. The activity of one new variant, FIX-LK, exceeds the activity of FIX-R338L by over 2 fold. In Aim 1, additional FIX variants will be identified and biochemically characterized with recombinant FIX protein. The in vivo efficacy of the best performing new variant will be comprehensively evaluated as protein and gene therapeutics in a HB mouse model in Aim 2. In Aim 3, the safety of the most promising new variants is assessed. The immunogenicity and thrombogenicity of new FIX variants is compared to wild-type FIX after provocative challenges in novel HB mouse models.